The disclosed invention is directed generally to head-up displays, and more particularly is directed to a head-up display which is controllable to provide high brightness during daytime and reduced ghost image intensity during nighttime.
Head-up displays are utilized in vehicles such as automobiles to produce virtual images of vehicle operating parameter indicators that appear to be ahead of the vehicle windshield and are therefore viewable by the vehicle operator without diversion of his or her eyes to an instrument panel inside the vehicle and without refocusing.
As a result of the thickness of the vehicle windshield and reflections at a glass/air interface that is not the primary reflecting surface, ghost images are produced. Depending on ambient lighting conditions and intensity relative to primary image intensity, ghost images can be objectionable.
The general requirements for head-up displays include sufficient image brightness that is not too bright, ghost image intensity that is not distractingly objectionable, and acceptable image contrast.
The foregoing general requirements for head-up displays typically require different specific characteristics for day and night use. Daytime use requires high image brightness, and permits higher ghost image intensity since the ghost images tend to blend with the ambient scene. Nighttime use requires lower primary image brightness, and lower ghost image intensity since ghost images are readily noticed against the generally dark ambient scene. While reducing imaging illumination intensity for night use provides for reducing image brightness, such reduction does not necessarily provide the sufficiently low ghost image intensity. In other words, the primary image to ghost image intensity ratio must be greater at night.